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A 45% Drain Efficiency, -5OdBc ACLR GaN HEMT Class-E Amplifierwith DPD for W-CDMA Base Station

Norihiko Ui and Seigo Sano

Eudyna Devices Inc. Yamanashi 409-3883, Japan

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Abstract - A IOW GaN HEMT Class-E amplifier for W-CDMA base station is demonstrated in this paper. We firstdemonstrate that a low pass type Class-E circuit has quite similarperformance to widely used series L-C resonance Class-E circuitby simulating voltage and current waveforms with 50V operationGaN HEMT at 2.1GHz. We achieved measurement results; drainefficiency of 82%, CW output power of 11W and Linear gain of19.5dB at 2.1GHz and 50V drain bias operation with a simple lowpass Class-E circuit. And ACLR of -50dBc was obtained with45% drain efriciency under 2-carrier W-CDMA , 7.8dB PAR(0.01%) signals, by using digital pre-distortion (DPD) operation.Index Terms - Class-E operation, GaN HEMT, microwave

frequency, digital pre-distortion(DPD)

I. INTRODUCTION

RF power efficiency is a key feature for high poweramplifiers in wireless or satellite communications. Recently,to obtain higher efficiency at microwave frequency, Class-Eoperation has been studied using Si LDMOS package devicesaround IGHz[1][2]. However, due to the large outputcapacitance (relative to low current density) of Si LDMOS,allowable frequency is limited at present. And peak drainvoltage in Class-E operation reaches approximately 3 times asmuch as DC drain voltage. Therefore high breakdown voltageis needed. In the recent few years, high current density andhigh breakdown voltage GaN HEMTs have been developedfor wireless communications [3] [4]. In this paper, wedemonstrate high power and high efficiency GaN HEMTamplifier with Class-E operation at more than 2GHz for W-CDMA BTS.

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Vds [V](a) I-V characteristics Vgs is from +2.OVto -2.OV with 0.5V step.

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Fig. 1. DC characteristics of GaN HEMT gate width of 1mm

II. Dc CHARACERISTICS

Class-E operation is basically a switching mode amplifier.Therefore low resistance and good off-state characteristics areneeded for the switching device. High breakdown voltage isalso needed as seen in section 3. Fig. 1(a) shows I-Vcharacteristics of a GaN HEMT with gate width of Imm[5].The device indicates high current density of 600mA/mm withlow on-resistance and very good pinch-off characteristics atmore than 200V with Vth of -1.7V. Furthermore, the gatebreakdown voltage is more than 350V as shown in Fig. 1(b)

III. WAVEFORM SIMULATION

A common Class-E amplifier configuration is shown inFig.2(a). In the ideal Class-E operation, synthesized currentwaveform of drain current (Id) and Cp current (Ic) is perfectlysinusoidal, because higher harmonics are filtered by the largeLs of the L-C series resonator. The Id and drain voltage (Vd)have zero-value alternately in a cycle. Therefore no harmonicand no DC power are dissipated in the circuit. In other words,all the applied DC power converts to fundamental RF power.To confirm the current and the voltage waveform, we

performed large signal simulation of the Id, Ic and Vdwaveform. Simulation results are shown in Fig.3(a) and (b)

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with the Cp of 0.6pF, the Ls of 7.5nH, the Cs of 1.8pF andGaN HEMT's gate width of 2.3mm at Vds=50V and Vgs=-1.4V at 2.IGHz and lOW output. The Cp is drain-sourcecapacitance (Cds) of the GaN HEMT itself. The Id and Ic areflowing alternately and their synthesized current is nearlysinusoidal. The Id and the Vd have about zero-valuealternately in a cycle. These simulated waveforms show aremarkable similarity to ideal Class-E waveform. Notice thatthe peak of Vds indicate the fairly high value of 135V,approximately 2.7(ideally 2.8) times as much as Vds=50V.But the Vgdo of the GaN HEMT is high enough at 350V tocope with the peak voltage.

Fig. 2. Common Bandpass Class-E amplifierconfiguration

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To realize a sinusoidal waveform, the series inductance isneeded to filter the higher harmonics. For that reason, L-Clowpass output matching network including enough highseries inductance for the harmonics is expected to substitutefor the series L-C resonator [6]. A lowpass type Class-Eamplifier configuration is shown in Fig.4 and its simulationresults are shown in Fig.5(a) and (b) with the Ls of 6.5nH andthe Cm of 1.lpF. The other conditions are same as the seriesresonator type. All the waveforms are quite similar to theseries resonator type. Therefore the low pass type Class-Eamplifier is acceptable for GaN HEMT at 2.1GHz and 50Voperation.

Fig. 4. Common Lowpass Class-E amplifierconfiguration

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Time[nSec.](a) Id and Vd waveform

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Id I.lc Id + Ic(b) Id, Ic and synthesized waveform

Fig. 3. Simulated waveform of the common bandpassClass-E amplifier at Pout=10W, Vd=50V and

Time[nSec.]

Id I.lc Id + Ic(b) Id, Ic and synthesized waveform

Fig. 5. Simulated waveform of the lowpass type Class-Eamplifier at Pout=10W, Vd=50V and freq=2.lGHz

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IV. CIRCUIT DESIGN

The IOW GaN HEMT Class-E amplifier circuit schematicinside and outside the package are shown in Fig.6(a) and (b).We employ single-stage input matching circuit of open-stubon a test fixture. The lowpass Class-E circuit composed of aparallel capacitor (Cp) same as Cds, a series inductance (Ls)and a matching capacitor (Cm) for the output are inside thepackage. Other output matching network is composed ofopen-stubs on the test fixture. A single cell GaN HEMT with agate width of 2.3mm is mounted on the package with a Cubased heat spreader. The Ls is composed of Au bonding wireswith an interconnection alumina substrate for connecting thewires.Advantages of this circuit topology are not only a flexibility

of a bias network connecting position on the output circuit, butalso to apply to higher power multi-cell GaN HEMT easily byconnecting GaN HEMT unit cells, drain bonding wires andmatching capacitors in parallel inside a package.

characteristics with digital pre-distortion (DPD) operation.The signal conditions are 2-carrier W-CDMA 7.8dBPAR(O.01%) signal, 5MHz spacing. 5MHz offset ACLR of -5OdBc was obtained with 45% drain efficiency at averageoutput power of 33.5dBm.

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Pin [dBm]Fig.7. Pin-Pout characteristics of 1OW GaN HEMTlowpass type Class-E amplifier at Vd=50V andfreq=2.1GHz

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Fig. 6. Circuit schematic of 1OW GaN HEMT lowpasstype Class-E amplifier

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V. MEASUREMENT RESULTS

Pin and Pout characteristics of the IOW GaN HEMT Class-E amplifier are shown in Fig.7. The output power of 11W,drain efficiency of 82%, associated power gain of 15.5dB atwith input power of 25dBm and linear gain of 19.5dB at Vdsof 50V and 2.1GHz was achieved. Fig.8 shows ACLR

Pout [dBm]Fig.8. ACLR characteristics of 1OW GaN HEMT low-pass typeClass-E amplifier at Vd=50V with digital pre-distortion (DPD)operation. The singal conditions are 2-carrier W-CDMA 7.8dBPAR(O.01 %) signal, 5MHz spacing.

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VI. RELIABILITY

Fig.9 shows the results of RF overdrive step stress test forthe lOW GaN HEMT Class-E amplifier at 2.1GHz with Vdsof 50V. The devices operated in 2 hours at each stress levelbetween 5dB to 15dB gain compression. No destructivefailures and degradations have been observed under theoverdrive stress test.

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REFERENCES[1] M. Akkul, M. Roberts, V. Walker and W. Bosch., "High

Efficiency Power Amplifier Input/Output Circuit Topologies forBase station and WLAN Applications" 2004 IEEE MTT-S Int.Microwave Symposium Digest, pp. 843 - 846

[2] F. Wang and D.B. Rutledge, "A 60-W L-Band ClassE/Fodd,2LDMOS Power Amplifier Using Compact Multilayered Baluns",2004 IEEE Topical Workshop on Power Amplifiers for WirelessCommunications.

[3] W. Nagy, et al., "150W GaN-on-Si RF Power Transistor",2005 IEEE MTT-S Int. Microwave Symposium Digest.

[4] K. Joshin, T. Kikkawa, H. Hayashi, T. Maniwa, S. Yokokawa,M. Yokoyama, N. Adachi, Takikawa., " A 174 W high-efficiencyGaN HEMT power amplifier for W-CDMA base stationapplications", 2003 IEEE IEDM Tech. Digest, pp. 983-985.

[5] J. Nikaido, T. Kikkawa, S. Yokokawa, and Y. Tateno.," AHighly Uniform and Reliable AlGaN/GaN HEMT", 2005 CSMANTECH Digest, pp. 151-154.

[6] Steve C. Cripps, "RF Power Amplifiers for WirelessCommunications", Artech House 1999.

Fig.9. RF overdrive step stress test of 1OW GaN HEMT Class-Eamplifier at Vds of 50V and 2.1GHz.

VII. CONCLUSIONS

We have demonstrated a lOW GaN HEMT Class-Eamplifier. Simulated maximum drain voltage reaches 135V,which is safely below the high breakdown voltage of 350V.We demonstrated that a lowpass type Class-E circuit is anacceptable circuit topology by simulating voltage and cuffentwaveforms. The output power of 11W with high drainefficiency of 82% and associated power gain of 15.5dB andlinear gain of 19.5dB was obtained at 2.1GHz for CWoperation. ACLR of -5OdBc was obtained with 45% drainefficiency with digital pre-distortion (DPD) operation. Theseresults will contribute to the design of higher power amplifiers,because this approach can be extended to multi-cell GaNHEMT devices.

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